This isn't quite feasible; you could only sell as many memberships as you had seats, and they'd have to eat there a LOT to recoup. Better to have a "members' section" that increases your chances of (but doesn't guarantee) getting seated.

Also guaranteed reservations for busy times would mean empty seats at busy times. I recommed the restaurant issues trump cards, so people can choose what level they want to pay for knowing that as they can "trump" people with lower memberships they too can be "trumped" by higher status cards. This way you have a small pool of reserved tables and a method to allocate them.

Maybe tables should just be auctioned off. That way, people who really want them can express the urgency of their need, no matter whether it's because they're in town for only one day, want to eat something before the show, or just because they're used to it.
Who kows, you might even pay for your meal by "getting bumped" and waiting half an hour longer.

An actual aution could be good. The trump card system I proposed, after all, is a defered auction and designed to be simple to implement at the the point where people haggle for an empty table. However people might not be too interested in a system that requires prior purchases and prefer an immediate auction. This might allow people to demonstrate how weathly they are, or how wealthy they wish to appear to be, which could be a bonus in some circles.

Somewhat baked (three-quarters-baked, then?) American Express boasts "Every day a reservation is held exclusively for Platinum Card members"; tables at select restaurants reserved for AMEXCO members until 10 a.m. day of feast. If no one's reserved those blocked-off tables by then, they're throw back in the pot.